This invention relates to an apparatus and method for normalizing a received radio signal.
Traditionally, the function of a radio receiver has been to detect, amplify and decode a received radio signal, while maintaining the best possible fidelity. However, in the present day environment, signal infidelity due to the presence of intentional or unintentional interference can cause a radio receiver to malfunction. The unintentional interferences are normally the consequence of radar, man-made disturbances such as the use of appliances and other modern electrical or electronic equipment, or other similar types of radio transmissions. The intentional interferences, which are usually encountered in military situations, may be from devices that are designed to interrupt or interfere with communications systems. These devices, commonly referred to as jammers, can render a tactical military receiver inoperative. In an effort to obviate the effects of both intentional and unintentional interference, different types of methods and apparatus have been incorporated in the design of communication transmitters and receivers. These include various methods of error detection, modulation schemes, precision tuning and filtering, automatic gain control, adaptive antenna array and spread spectrum communication systems.
Many of the aforementioned methods and apparatus and combinations thereof have been used in the prior art to enhance the overall performance of a communication system and reduce the undesirable effects caused by interference with the transmission of radio signals. For example, in a recent article, "Adaptive Arrays--An Introduction", by William F. Gabriel, Proceedings of the IEEE, Vol. 64, No. 2, February, 1976, the use of an adaptive antenna array system was discussed for radar and communications applications. In this area, use of adaptive antenna array systems in large stationary systems are successful, especially since adaptive antenna array systems provide the ability to automatically steer nulls onto undesirable sources of interference, and thereby reduce output noise and enhance detection of the desired signal. The systems that utilize adaptive antenna array systems usually consist of an array of antenna elements and a real time adaptive receiver processor which has feedback control over the elements' weights. However, as obvious from the above-mentioned reference, adaptive antenna array systems require the application of a large servo control system for steering the antenna array.
Although adaptive antenna arrays can readily be implemented on large complex stationary radar and communication systems, they hardly lend themselves for implementation on the smaller radio receivers that, in many cases, are mobile. Because of this, there is a definite need for a method and apparatus for overcoming interference of both wide-band and narrow-band sources, especially those that are caused by amplitude modulation of a desired signal with a jamming signal that results in either a wide-band interference, narrow-band interference, or both, having a slowly varying amplitude modulation envelope. The signal normalizer, as disclosed herein, has been effective in reducing by 20 dB the effects of interference signals, especially those that are derived from a wide-band jammer.